Who is the housebuilder?
You should write to the NHBC and ask that this builder is de-registered from the NHBC as they clearly are breaching their requirements under the NHBC warranty. You should also ask the NHBC to appoint a contractor to carry out the work required.
It is perfectly reasonable and advisable to write to the builder and report all defects as and when they become evident. Even if they are not dealt with within the 2-year warranty period, the house builder is still on the hook if they are reported within the first 2 years.